Air pollution is a worldwide concern and many countries have enacted stricter laws further limiting the emission of pollutants from gas turbine engines or offer fiscal or other benefits for environmentally sound installations. One method for reducing the emission of pollutants is thorough mixing of fuel and air prior to combustion which prevents high temperature stoichiometric fuel air mixtures in the combustor. Therefore the temperature dependent formation rate of NOx is lowered. Although the prior techniques for reducing the emissions of NOx from gas turbine engines are steps in the right direction, the need for additional improvements remains.
There are two main measures by which reduction of the temperature of the combustion flame can be achieved. The first is to use a fine distribution of fuel in the air, generating a fuel/air mixture with a low fuel fraction. The thermal mass of the excess air present in the reaction zone of a lean pre-mixed combustor absorbs heat and reduces the temperature rise of the products of combustion to a level where thermal NOx is not excessively formed. The second measure is to provide a thorough mixing of fuel and air prior to combustion. The better the mixing, the fewer regions exist where the fuel concentration is significantly higher than average, the fewer the regions reaching higher temperatures than average, the lower the fraction of thermal NOx will be.
Usually the premixing takes place by injecting fuel into an air stream in a swirling zone of a combustor which is located upstream from the combustion zone. The swirling produces a mixing of fuel and air before the mixture enters the combustion zone.
US 2001/0052229 A1 describes a burner with uniform fuel/air premixing. The premixer includes vanes that impart swirl to the airflow entering via the compressor air inlet openings. Each vane contains internal fuel flow tubes that introduce natural gas fuel into the air stream via fuel metering holes that pass through the walls of the vanes.
U.S. Pat. No. 5,511,375 describes an axial swirler having vanes containing internal concentric passages of flow exiting through holes near the trailing edge. The centre passage contains liquid fuel and the surrounding passage gaseous fuel. The arrangement is intended for a dual fuel burner.